1. Field of the Invention
The present invention relates to a developing unit for use in an electronic printing apparatus. More particularly, the present invention relates to a developing unit which is capable of easily adjusting a gap between a doctor blade and magnetic roller and stably maintaining this gap.
2. Description of the Related Art
In an electronic photographic printer or electronic copying machine, an electrostatic latent image corresponding to an image to be printed or copied is optically formed on a photosensitive drum. Thereafter, the electrostatic latent image is developed using powdered toner and the image produced by the toner is transferred to recording paper and fixed. Thus, a desired image is printed or copied. FIG. 1 is a schematic view illustrating the construction of such an electronic photographic printer. The electronic photographic printer comprises a photosensitive drum 1 having a cylindrical surface on which electrostatic latent images are formed by projecting optical images; a cleaner 2 for cleaning the cylindrical surface of the photosensitive drum 1; a charging unit 3 for uniformly charging the cylindrical surface of the photosensitive drum 1 by ultraviolet-ray irradiation or the like; and a scanning unit 4 for scanning the cylindrical surface of the photosensitive drum 1 with a light beam RB from a laser diode (not shown) or the like. The scanning unit 4 comprises a polygonal mirror 4a for causing the light beam RB to deflect in the axial direction of the photosensitive drum 1; a focal quality fg lens 4b for compensating for a focal distance; a mirror 4c for deflecting light beam RB; and a cylindrical lens 4d for making the light beam RB converge on the cylindrical surface of drum 1.
When the light beam RB is turned on/off on the basis of image data while the photosensitive drum 1 is being rotated and the cylindrical surface of the photosensitive drum 1 is being scanned with the light beam RB, an electrostatic latent image is formed on the cylindrical surface of the photosensitive drum 1.
In FIG. 1, reference numeral 5 denotes a developing unit for developing the electrostatic latent image by toner; reference numeral 6 denotes a transfer unit for transferring the image formed by toner, transferred onto the cylindrical surface of the photosensitive drum 1, to recording paper; reference numeral 7 denotes a discharging unit for removing charges remaining on the recording paper; reference numeral 8 denotes a fixing unit for fixing toner to recording paper; and reference numeral 9 denotes a hopper for recording paper, for example, two hoppers 9a and 9b for supplying sheets of paper of different sizes are provided; reference numeral 10 denotes a guide for feeding recording paper; reference numeral 11 denotes a stacker for recording paper; and reference numeral 12 denotes a top lid.
As shown in FIG. 2, the developing unit 5 comprises a container 51 which is filled with powdered toner TN and a frame 52 for holding the container. The container 51 is generally divided into a stirring section 5a for causing frictional electrification of the particles of toner TN by stirring the toner, and a toner-separation section 5b. A sensor 5c for detecting the amount of the toner TN remaining inside the container 51 is disposed in the vicinity of the bottom thereof.
The toner TN is stirred as a result of a stirring member 5a-1 being rotated. The granular toner TN is electrified by the friction between the toner TN and the stirring member 5a-1, and sent to the toner-separation section 5b. The electrified toner TN adheres by a magnetic force to a magnetic roller 5b-1 in the toner-separation section 5b. In this way, the toner particles adhere to the cylindrical surface of the magnetic roller 5b-1 in a layered form. After the thickness of the toner particles TN, i.e., the height from the cylindrical surface, is restricted by a doctor blade 5b-2 placed in proximity to the cylindrical surface of the magnetic roller 5b-1, the toner particles contact the cylindrical surface of the photosensitive drum 1. As a result, the toner TN moves from the magnetic roller 5b-1 to the photosensitive drum 1. However, the amount of the toner TN moved varies in proportion to the magnitude of the difference between a bias voltage applied to the magnetic roller 5b-1 and the electrical potential resulting from the electrostatic latent image on the cylindrical surface of the photosensitive drum 1. Only those electrostatic latent images in which the difference between the bias voltage and the electrical potential is larger than a threshold value are developed by the toner TN.
In the conventional developing unit 5, both the magnetic roller 5b-1 and the doctor blade 5b-2 are directly fixed to the frame 52. As a lighter weight electronic photographic printer or electronic copying machine is demanded, the frame 51 is usually formed from plastic materials. As a consequence, the rigidity thereof is slight, and therefore distortion is likely to occur when an external force is applied thereto. When, for example, a printer is placed on a base which is not flat, the entire chassis and casing of the printer is deformed due to the weight of the printer, say, 30-50 kg, causing the frame 52 of the developing unit 5 to be distorted. The gap between the magnetic roller 5b-1 and the doctor blade 5b-2 is as narrow as 0.26 to 0.28 mm, and is easily caused to vary due to such distortion of the developing unit 5 as that described above. Therefore the height of the toner TN on the cylindrical surface of the magnetic roller 5b-1 is no longer maintained at a constant. A problem arises in that the amount of the toner TN transferred to the cylindrical surface of the photosensitive drum 1 changes with not only the potential difference between the magnetic roller 5b-1 and the drum 1 but also with the change of the gap. When the gap is not uniform, the density of the image on the recording paper is not uniform. Also, variations in the gap are sometimes caused by vibrations from outside. If such problems occur, complicated and complex adjustments and maintenance of electronic photographic printers or electronic copying machines must be performed.